1. Field of the Invention
The present invention relates to speed change control systems of automotive automatic transmissions. More specifically, the present invention relates to the speed change control systems of a type that is constructed to suppress or at least minimizes a select shock of the transmission that would occur when a select operation takes place under a higher load condition of an associated engine.
2. Description of the Related Art
In order to clarify the task of the present invention, the related art of the speed change control systems of such type will be described before describing the details of the invention.
In general, in an automotive automatic transmission including a planetary gear unit and a torque converter, there are employed various friction elements such as hydraulic multiple disc clutches, hydraulic brakes and the like. Upon shifting, some of the friction elements are selected by a controller and brought into their engaged condition providing the transmission with a desired speed or a reverse.
That is, for example, when, with an associated motor vehicle being at a standstill, a select lever is shifted from N (neutral)-range to D (drive)-range (which will be referred to as “N-D selection” hereinafter), the controller brings a first-speed corresponding clutch into an engaged condition to establish 1st speed in the transmission. In this case, the hydraulic pressure for the 1st speed corresponding clutch is feedback controlled by the controller in such a manner that a rate of change “dNt” of an input shaft rotation speed “Nt” has a target value “dNti” and the input shaft rotation speed is reduced to a synchronized 1st speed “N1”. Usually, the target rate of speed change “dNti” is set to such a fixed value as to produce no select shock and establish the speed change operation as fast as possible. The fixed value has been previously stored in a memory of the controller. When it is detected that the input shaft rotation speed “Nt” shows the synchronized 1st speed “N1”, the controller engages the 1st speed corresponding clutch completely thereby to complete the shift change to 1st speed.
Measures for improving the operation of such shift change of N-D selection are described in Japanese Laid-open Patent Application (Tokkaihei) 5-322027. The measures disclosed by this published application pay attention to a change in time that is needed for completing the shift change in accordance with the input shaft rotation speed “Nt” at a start timing of the shift change in case wherein the target rate of speed change “dNt” of the input shaft is set to a fixed value at the time of the N-D selection. In the measures, for making the time needed for completing the shift change constant irrespective of the rotation speed of the input shaft at the N-D selection, the target rate of speed change “dNt” of the input shaft is set in accordance with the input shaft rotation speed “Nt” at the start timing of the N-D selection.
According to the measures, the time needed for completing the shift change at the N-D selection can have a generally constant value, and thus, a shift feeling is fairly improved.
For actuating the clutches, a hydraulic power system is used. That is, each clutch is connected to a hydraulic power source through an oil passage and an electromagnetic valve (or solenoid valve) is installed in the oil passage. For controlling the hydraulic pressure fed to the clutch, the electromagnetic valve is subjected to a duty-control.
However, due to the inherent construction of the hydraulic power system, a feedback control for the clutches produces a pulsation in the fluid in the oil passage due to ON/OFF operation of the electromagnetic valves, and thus a hydraulic fluctuation is inevitably produced. Usually, for treating such hydraulic fluctuation, accumulators are provided for respective clutches in the hydraulic circuit.